PHP Calculation - Modulus 10 check digit generation

Question

I got a problem here figuring out how to do this:

1. I got a document no for example 5843
2. Starting with the right-most digit, multiply every other digit by 2 and 1 (For example, (3x2)+(4x1)+(8x2)+(5x1) and so on)
3. For every multiplication that results a number more than 9, add them digits. (For example, from the calculation on (no.2) we will get this : (6)+(4)+(16)+(5). So we got a number (16) which is more that 9. We need to add them (1+6=7). Now the output will be like this : (6)+(4)+(7)+(5).
4. Next, we add them (6)+(4)+(7)+(5)=22
5. Now, we divide 22 by 10 and get the remainder which is 2 in this case.
6. Lastly we minus the remainder with 10.
7. So the last output will be 8

Can you guys guide me on how to do this? Thank you very much !

Answer 1

In case someone gets to the question wondering how to use modulus or 'getting the rest of a division' (%):

echo 5 % 3; // outputs 2

---

(You can take a look at a step-by-step output after this first part)

This is the commented function:

function func_name($docnum){

    // make sure there is just numbers in $docnum
    $docnum = preg_replace("/[^0-9]/","",$docnum);      

    // change order of values to use in foreach
    $vals = array_reverse(str_split($docnum));

    // multiply every other value by 2
    $mult = true;
    foreach($vals as $k => $v){
        $vals[$k] = $mult ? $v*2: $v;
        $vals[$k] = (string)($vals[$k]);
        $mult = !$mult;
    }

    // checks for two digits (>9)
    $mp = array_map(function($v){
        return ($v > 9) ? $v[0] + $v[1] : $v;
    }, $vals);

    // adds the values
    $sum = array_sum($mp);

    //gets the mod
    $md = $sum % 10;

    // checks how much for 10
    $result = 10 - $md;

    // returns the value
    return $result;
}

---

Some test runs:

$docnum = 5843;
echo func_name($docnum);

Output:

8

---

$docnum = 1111;
echo func_name($docnum);

Output:

4

---

$docnum = '-5a84fadsafds.fdar3';
echo func_name($docnum);

Output:

8

---

$docnum = 4321;
echo func_name($docnum);

Output:

6

---

This codes outputs the steps...

$docnum = '-5a84fadsafds.fdar3';

echo 'Original Value: ';
print_r($docnum);
echo '<hr>';

$docnum = preg_replace("/[^0-9]/","",$docnum); 

echo 'Numbers only: ';
print_r($docnum);
echo '<hr>';

$vals = array_reverse(str_split($docnum));

echo 'Reversed Value: ';
print_r($vals);
echo '<hr>';

$mult = true;
foreach($vals as $k => $v){
    $vals[$k] = $mult ? $v*2: $v;
    $vals[$k] = (string)($vals[$k]);
    $mult = !$mult;
}

echo 'After mult.: ';
print_r($vals);
echo '<hr>';

$mp = array_map(function($v){
    return ($v > 9) ? $v[0] + $v[1] : $v;
}, $vals);

echo 'Checked for >9: ';
print_r($mp);
echo '<hr>';

$sum = array_sum($mp);

echo 'All values together: ';
print_r($sum);
echo '<hr>';

$md = $sum % 10;

echo 'Mod: ';
print_r($md);
echo '<hr>';

$result = 10 - $md;

echo 'Final result: ';
print_r($result);
echo '<hr>';

Output:

Original Value: -5a84fadsafds.fdar3
Numbers only: 5843
Reversed Value: Array ( [0] => 3 [1] => 4 [2] => 8 [3] => 5 )
After mult.: Array ( [0] => 6 [1] => 4 [2] => 16 [3] => 5 )
Checked for >9: Array ( [0] => 6 [1] => 4 [2] => 7 [3] => 5 )
All values together: 22
Mod: 2
Final result: 8

---

Read more about things used in the code:

• http://php.net/manual/en/function.array-reverse.php
• http://php.net/manual/en/function.array-map.php
• http://php.net/manual/en/function.array-sum.php
• http://php.net/manual/en/control-structures.foreach.php
• http://php.net/manual/en/language.operators.arithmetic.php (modulus)
• http://php.net/manual/en/language.types.type-juggling.php ((string))

---

As suggested by @Floris, you can replace the double-digit sum part with modulus 9, so instead of

$mp = array_map(function($v){
        return ($v > 9) ? $v[0] + $v[1] : $v;
    }, $vals);

you would have

$mp = array_map(function($v){
        return $v % 9;
    }, $vals);

and still keep the same output...

Answer 2

So u just want a function to calculate your stuff? i would just convert your integer to a string and then do your calculation number by number. So it would look like this:

<?php

$NUMBER = 5843;

echo CalculateThing($NUMBER); // output: 8

function CalculateThing($inputNumber)
{
    $output = 0;
    $number = (string)$inputNumber;
    $mulTwo = true;
    for($index=strlen($number)-1; $index>=0; $index--) {
        $val = $mulTwo ? intval($number[$index]) * 2 : intval($number[$index]);
        if($val>9){
            $val = (string)$val;
            $val = $val[0]+$val[1];
        }
        $mulTwo = !$mulTwo;
        $output += intval($val);
    }
    $output = (($output % 10)-10)*-1;
    return $output;
}